The present invention is related to xerographic copying systems and, more particularly, to systems which employ what is known as "transfer" or "touchdown" development.
The xerographic process as disclosed in Carlson's patent U.S. Pat. No. 2,297,691, encompasses a xerographic plate comprising a layer of photoconductive insulating material on a conductive backing. This plate is provided with a uniform electric charge over its surface and is then light exposed to the subject matter to be reproduced. The light exposure discharges the plate areas in accordance with the radiation intensity that reaches it and thereby creates a latent electrostatically charged image on or in the photoconductive layer. Development of the latent image is effected with an electrostatically charged, finely divided material, such as an electroscopic powder, that is brought into surface contact with the photoconductive layer and is held thereon electrostatically in a selective pattern corresponding to the latent electrostatic image. Thereafter, the developed image may be fixed by any suitable means to the surface on which it has been developed or the developed image may be transferred to a secondary support surface to which it may be fixed or utilized by means known in the art.
Once the electrostatic latent image is formed, the method by which it is made visible is the developing process. Various developing systems are well known in the art and include cascade, brush development, magnetic brush, powder cloud and liquid development. Still another developing method is disclosed in Mayo patent U.S. Pat. No. 2,895,847 in which a developer support member, called a "donor," is employed to present a releasable layer of electroscopic (toner) particles to the photoconductive layer for deposit thereon in conformity with the electrostatic latent image. The Mayo approach is one of several variations which involve the transfer of toner particles from a donor to the photoconductive surface and is therefore called transfer development. This technique is also known as "touchdown development."
The three principal variations of transfer development include (1) an arrangement in which the layer of toner on the donor surface is held out of contact with the electrostatically imaged photoconductor and the toner must traverse an air gap to effect development; (2) an arrangement in which the toner layer on the donor is brought into rolling contact with the imaged photoconductor; and (3) an arrangement in which the toner layer is brought into contact with the imaged photoconductor and skidded across the imaged surface to effect development.
In the first of the above arrangements where the toner and photoconductor surface are maintained out of contact, a layer of toner particles is applied to a donor member which is capable of retaining the particles on its surface and then the donor member is brought into close proximity to the surface of the photoconductor. In this closely spaced position, particles of toner in the toner layer on the donor member are attracted to the photoconductor by the electrostatic charge on the photoconductor so that development can occur. Typically, the spacing between donor and photoconductor is between 1 and 10 mils. This arrangement is referred to as "spaced touchdown development."
In touchdown development, a variety of donor is possible and known in the art. A donor member may be constructed of a variety of materials which includes paper, plastic, cloth, metal, aluminum foil or metal-backed paper.
In U.S. Pat. No. 3,203,394 to Hope et al., various donors are described which employ the principle of using a set of conductive posts or a conductive screen which is charged in the same polarity and selective amount as the charged toner particles. Accordingly, as the donor member is brought into contact with the toner particles, those areas adjacent to the posts or screen will electrostatically repel the toner, thereby forcing the toner away from those portions. The remaining areas of the donor member are charged to attract the toner particles and the particles accumulated there. As described in the Hope et al. patent, a donor member of this type of construction provides better mobility to the toner particles so as to yield sharper xerographic copies.
In U.S. Pat. No. 3,375,806 to Nost, the donor member is described as being either electrically insulative or conductive and may comprise such materials as metal sheets, conductive rubbers, Mylar or the like.
Although spaced touchdown may be used with a variety of donor as discussed above, certain problems exist in this approach. One of the problems of the spaced donor arrangement is the difficulty of maintaining the aforementioned spaced relationship between the donor surface and the photoconductive surface. Additionally, in all transfer development systems, uniform deposition of toner onto the donor, which is a requirement for high quality prints, has been difficult to achieve because of the tendency of toner to clump and because of the internal electrostatic forces among the toner particles.
One approach for obviating the above problems has been the use of a donor member having a surface with raised and depressed portions, such as a gravure surface with an elevated grid network enclosing a plurality of depressed cups, as disclosed in Greig, U.S. Pat. No. 2,811,465. If such a donor member were used in contact with the imaging surface and doctored such that toner resided only in the cups, theoretically the toner would not contact the background, or uncharged, areas of the imaging surface. That is, the uniform gap between toner and image could be maintained by having the raised areas of the donor as the only point of contact on the imaging surface. Toner would, of course, still be attracted from the depressed portions of the donor to the charged, imaged areas, but the need for complicated, gap-controlling means would be eliminated. Additionally, the roughened surface would tend to break up clumps of toner during the loading step.
However, in practice, although such a donor member produced somewhat improved transfer development, it was found that toner could not be efficiently loaded on the donor without at least partially covering the raised grid structure with toner. Thus, toner was still contacting background areas of the imaging surface, thereby producing some background deposition in the copy. Also, the images produced bore the impression of the grid structure due to interference of the grid with complete toner deposition on the charged areas. Clearly, both the above results are undesirable in a high-quality imaging process.